Internal arc limiting coatings for cathode ray tubes have long been known. Such coatings have been utilized to provide a discharge path for secondary electrons emitted by the perforated mask when impinged by the primary electron beam. Normally, such coatings have been of the so-called "soft" aquadag variety which are primarily graphite dispersed in a binder material. Moreover, the coating is disposed intermediate a viewing screen, an anode button electrode and a mount assembly sealed into the envelope.
However, the appearance of solid state circuitry added complexity to the arcing problem because of the tendency toward catastrophic failure of the semiconductors of the solid state circuitry whenever a high current or arc current was transmitted to the circuitry by way of the internal coating and high voltage anode button. Thus, inhibition of currents developed by undesired arcing within the cathode ray tube became necessary and a replacement for the so-called "soft" aquadag coating was developed.
One form of replacement coating is an arc resistive coating utilizing a glass frit and a conductive oxide such as cadmium or copper oxide. This arc resistive coating provides a relatively high resistance whereby arc currents are limited and catastrophic semi-conductor failures inhibited. Moreover, such a coating is set forth and discussed in U.S. Pat. No. 4,124,540 issued to Foreman et al. on Nov. 7, 1978 and assigned to the Assignee of the present application.
Although the above-mentioned "soft" aquadag and so-called "frit" type coatings have been and still are utilized in some applications with excellent results, it has been found that there are other applications wherein such coatings leave something to be desired. Moreover, the "soft" aquadag coatings tend toward undesired loose particles and arcing while the so-called "frit" type coatings are most difficult to salvage and are expensive of labor and materials.
In an effort to reduce the above-mentioned arcing problems and provide a salvageable but relatively inexpensive resistance coating, the so-called iron oxide type coatings were developed. As exemplified by U.S. Pat. No. 3,791,546 issued to Maley et al., an iron oxide, graphite and alkali silicate coating was provided. This coating had an oxide to graphite ratio in the range of about 2:1 to 6:1 which unfortunately tended to provide an excessive amount of graphite particles and these excess graphite particles became loose and tended to cause arcing.
Additionally, it has been found that the iron oxide mixed with the graphite tends to undesirably reduce due to the relatively high temperatures produced by the electron beam impingement occurring in a cathode ray tube. Unfortunately, the iron oxide is reduced to a lower oxide or even a metallic iron producing carbon monoxide and carbon dioxide. Such a reaction is indicated by the formulation: EQU Fe.sub.2 O.sub.3 +2C.fwdarw.FeO+Fe+CO+CO.sub.2
Thereafter, the carbon monoxide or carbon dioxide oxidizes any free barium at the surface of the cathode of the cathode ray tube to provide barium oxide. EQU CO+Ba.fwdarw.C+BaO EQU CO.sub.2 +2Ba.fwdarw.C+2BaO
This depletion of the free barium at the surface of the cathode undesirably raises the work function and lowers the electron emission for a given amount of power input. As a result of this reduced emission, premature and often times catastrophic tube failure is encountered.